The present invention relates to a pneumatic radial tire for a passenger car improved in the driving stability with a reduction in the weight of the tread belt structure.
Environmental pollution spreading on an earth scale in recent years has brought about a strong demand for a further lowering in the fuel consumption of vehicles, and, as a part of it, a reduction in the weight of the tire has become brought to the fore as a large technical task.
In a pneumatic radial tire for a passenger car, it is known that a high driving stability can be attained when the belt layer comprises steel cords because the steel cords are far superior in the strength and the modulus of elasticity to other fiber cords. The steel cords, however, unfavorably increase the weight of the tire and the fuel consumption due to its high specific gravity, so that it is difficult for them to cope with the above-described technical task.
Aramid fiber cords have been proposed as a tire cord material having properties close to the steel cords. They have a high strength and a modulus of elasticity comparable to those of the steel cords and can contribute to a reduction in the weight of the tire because the specific gravity is smaller than that of the steel cords. For example, it is known that the weight of the tire can be reduced by as large as about 5 to 8% through a mere replacement of the steel cords of the belt layer with the aramid fiber cords.
Since, however, the compressive rigidity of the aramid fiber cords is substantially equal to zero, the bending rigidity when a bending deformation is applied is disadvantageously low. For this reason, the cornering power in the case where the belt structure is the same as the belt structure comprising steel cords except that the belt cords are replaced with aramid fiber cords is as low as 75% of the cornering power attained by the belt layer comprising steel cords. Therefore, an attainment of the driving stability comparable to that of the tire having a belt structure comprising steel cords has been viewed as practically impossible when use is made of aramid fiber cords.